Fault detection circuit for use with a power control device

ABSTRACT

A fail open circuit that includes a controllable switch coupling a power source to a load. Control circuitry is provided that determines if the switch is in the proper conduction state based on a switch control signal and a signal indicative of power delivered to the load. If the switch is determined as improperly closed (conducting), the control circuitry diverts energy delivered to the load through fuse circuitry, thereby blowing a fuse and decoupling the load from the power source. In preferred embodiments, logic circuitry determines the relative states of the control switch and the load and generates a control signal to divert energy away from the load and blow a fuse.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a fail open circuit for use witha power control device to detect a short circuit in the power controldevice. Particularly, the fail open circuit detects a shorted powercontrol device by comparing the power input current and a power outputcurrent when the power control device is intended to be off. Further,the present invention uses the output of the power control device itselfto open the respective circuit if that power control device is shorted.The present invention is applicable to DC or AC power switchingapplications. Also, a typical power control device is a solid statepower switch, although the present invention is not limited to such aspower control device.

[0003] 2. Description of Related Art

[0004] Solid state power switches used as power control devices tend tofail in shorted states, which is often unacceptable in power switchingapplications. Various fail open circuits for detecting such shortcircuits in power switching applications are known. In one suchtechnique, a second solid state power switch device is used to turn offthe current when a primary solid state power switch fails to respond.This method is not failsafe, since both devices may be shorted by onelarge power surge. In another technique, a mechanical fuse or circuitbreaker is added in series with the solid state power switch device.This technique is based on the assumption that when the solid statepower switch device is shorted, so is the load. This is not always thecase. Therefore, this technique fails to detect a shorted solid statepower switch where the power switch is shorted but the load is not.

[0005] In yet another technique, a thermal/current fuse is placed inclose proximity to a power control device such that the fuse opens thecircuit when the solid state power switch carries the full load current,thereby overheating the fuse, or in the event that the temperature ofthe power switch exceeds the fuse's temperature rating. This techniqueis effective even where the power switch is shorted but the load is not.However, this technique is not failsafe in that it requires the powercontrol device itself to exceed the fuse's temperature or current ratingin order for the circuit to be opened.

SUMMARY OF THE INVENTION

[0006] The present invention solves the aforementioned drawbacks of theprior art by providing a fail open circuit that compares the conductionstate of a switch coupling power to a load, and if the switch isimproperly closed the present invention utilizes the energy delivered tothe load to blow a fuse and decouple the load from a power source. Inone embodiment, the present invention provides a fail open circuitcomprising an input voltage coupled to an output load through a controlswitch; a control signal regulating the conduction state of said controlswitch; and fail open circuitry receiving said control signal and asignal indicative of the energy delivered to said load, and adapted todetermine if said control switch is in the proper conduction state basedon said control signal, said fail open circuit further adapted todecouple said input voltage from said load using said energy deliveredto said output load if said control switch is in an improper conductionstate.

[0007] In method form, the present invention provides a method forfail-open circuit operation, comprising the steps of:

[0008] coupling an input voltage to a load through a control switch;

[0009] regulating the conduction state of said control switch with acontrol signal;

[0010] determining if said control switch is in a proper conductionstate based on said control signal and the energy delivered to saidload;

[0011] decoupling said input voltage from said load using said energydelivered to said load if said control switch is in an improperconduction state.

[0012] It will be appreciated by those skilled in the art that althoughthe following Detailed Description will proceed with reference beingmade to preferred embodiments and methods of use, the present inventionis not intended to be limited to these preferred embodiments and methodsof use. Rather, the present invention is of broad scope and is intendedto be limited only as set forth in the accompanying claims.

[0013] Other features and advantages of the present invention willbecome apparent as the following Detailed Description proceeds, and uponreference to the Drawings, wherein like numerals depict like parts, andwherein:

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a circuit diagram of an exemplary power switchingcircuit of the present invention including an exemplary fail opencircuit and solid-state power switch;

[0015]FIG. 2 is a chart of inputs and outputs of the components ofheater control circuitry in a preferred embodiment of the presentinvention.

[0016]FIG. 3 provides a chart in summary of the conditions and resultsof the preferred embodiment of fail open circuit 100, illustrating theconditions for opening the circuit.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0017]FIG. 1 is circuit diagram of an exemplary power switching circuit10, including a power input 24, control voltage 22, power control device(switch) 28, an exemplary fail open circuit 100, and power output 40.The power input 24 supplies power to the circuit, and comprises one ormore AC or DC sources. The control voltage 22 turns on or off switch 28either allowing or disallowing, respectively, current from the powerinput 24 to be transmitted to the power output 40.

[0018] In order to ensure fail-open operation of the circuit 10, a failopen circuit 100 with control circuitry 35 is provided to determine whento open the circuit 10 based on the condition of the control voltage 22,and the conduction state of the switch 28. In the example shown in FIG.1, the fail open circuitry 100 includes fuse circuitry 30 for openingthe circuit 10. Control circuitry 35 monitors both the control voltage22 and the output condition to determine if the switch 28 is incorrectlyconducting (i.e., failed shorted), thereby commanding fuse 30 to blow.The exemplary circuits 35 and 100 are described in greater detail below.

[0019] Fuse circuitry 30 comprises a heating resistor 31 and a thermalfuse 32 for opening the circuit 10. To determine the output condition anoutput sense resistor R_(o) 37, a current shunt and amplifier 38 areprovided. The signal developed across resistor 37 is fed into thecontrol circuitry 35 to control the conduction state of the fuse controlswitch 20, as described below. Thus, the present invention uses theenergy delivered to the load to effectuate blowing the fuse.

[0020] It should be noted that the following description assumes thatswitch 28 is active (conducting) high, but it will be apparent that thepresent invention can likewise be adapted to operate with active lowswitches. Switch 28 is in an improper conduction state if it conductswhen control voltage 22 commands that switch 28 be open. That is, switch28 is in an improper condition when it is shorted. In the preferredembodiment, the control voltage 22 delivers a high/on signal to theswitch 28 to command the switch to close, and a low/off signal tocommand the switch to open. In this embodiment, note that, if thecontrol voltage 22 is off, irrespective of the status of the powerinput, the switch 28 should be open, and therefore, conduction throughthe switch indicates an improper conduction state. Fail open circuit 100opens the circuit 10 when the control voltage 22 is off and the switch28 conducts.

[0021] Control circuitry 35 determines whether the above condition issatisfied, and generates a signal that causes the opening of the circuit10. In the preferred embodiment, circuitry 35 receives as inputs,signals determinative of whether the control voltage 22 is on, anddeterminative of whether power is being supplied to the output 40 of thecircuit 10. The latter input is supplied by the output of theaforementioned current shunt and amplifier 38. In the preferredembodiment, current shunt and amplifier 38 respectively diverts andamplifies current through output sense resistor, R_(o) 37, in serieswith the power output 40. Each of the inputs is received by circuitry 35as a high or low, i.e., binary 1 or 0, signal determining whether thatsignal is on or off respectively. This is discussed below.

[0022] Preferably, control circuitry 35 comprises logic devicesincluding an inverter 36 and an AND gate 34 in the configurationillustrated by FIG. 1. Inverter 36 generates the compliment of the inputsignal from control voltage 22. That is, inverter 36 generates a highsignal when control voltage 22 commands that switch 28 be open. Gate 34performs an AND operation on the above result of the inverter 36 and onthe output of current shunt and amplifier 38. That is, the result ofgate 34 is the result of output of control circuitry 35. In thepreferred this output is a high signal when control voltage 22 islow/off, and output power is still on.

[0023]FIG. 2 provides a table of binary inputs and outputs of inverter36 and AND gate 34, and of the preferred embodiment of circuitry 35. Asseen from the illustration, the output of the circuitry 35, i.e., outputof AND gate 34, is high when both inputs of the AND gate 34 are high.This condition is satisfied if power is delivered to the output 40 ofthe circuit 10 and the output of the inverter 36 is high. The output ofthe inverter 36 is high when control voltage 22 to the switch 28 islow/off. In alternative embodiments and based on alternative controlsignal configurations for switch 22, control circuitry 35 may compriseother components to generate a commanding signal causing the circuit 10to open when switch 28 is in an improper conduction state.

[0024] Returning to FIG. 1, in the preferred embodiment, when powerswitch 28 is shorted and the output of the control circuitry 35 is highas discussed above, the output of power switch 28 is used to open thecircuit 10. As seen in FIG. 1, the fuse control switch 20 receives theoutput of control circuitry 35. Generally, fuse control switch 20functions as a current sink upon being commanded on by a signal fromcircuitry 35. Preferably, device 20 is a PNP transistor, with the outputof circuitry 35 received at its base, or gate, and allowing current tosink to ground upon receiving a high signal from circuitry 35.Alternatively, device 20 may be an NPN transistor triggered by a lowinput at its base or gate. Of course, such an alternative embodimentrequires an alternative configuration of logic components in the controlcircuitry 35 to deliver a low signal when the power switch 28 is in animproper conduction state. In the preferred embodiment, when device 20receives a high signal from circuitry 35, current is sunk from theoutput of switch 28, thereby powering heating resistor 31 in series withthe fuse control switch 20. Resistor 31 heats a thermal fuse 32, inseries with switch 28 and power output 40, and placed in thermalproximity to the resistor 31. In turn, in the preferred embodiment,thermal fuse 32 exceeds its temperature rating and blows, openingcircuit 10 and preventing unwanted power from being delivered at thepower output 40. It will be apparent to one skilled in the art thatalternatively, any device, which generates heat upon receiving power,may replace resistor 31.

[0025]FIG. 3 provides a flowchart 200 in summary of the conditions andresults of the preferred embodiment of fail open circuit, illustratingthe conditions for opening the circuit. For clarity reference will bemade to the components of FIG. 1 without reference numerals. Initially,fail open circuit checks the status of control signal 202, where, in thepreferred embodiment, control signal is on when it commands controlswitch to be closed, or off when it commands control switch to be open.If the control signal is off, then power to the output load is checked204. If power exists at the load, then the thermal fuse is blown 206,decoupling the power input from the load. That is, in the abovecondition, switch 28 is determined to be in an improper conductionstate, i.e., shorted, and the circuit is opened. Additionally, the fuseblows when the current output of the control switch exceeds the currentrating of the fuse 208. Of course, power input is on when this conditionoccurs. Further, fuse is placed in proximity to switch such that fuseblows when the switch itself exceeds the fuse's temperature rating 210.These are secondary measures provided in addition to the primaryfunction of fail open circuit in the preferred embodiment of the presentinvention.

[0026] Alternative embodiments of the present invention allow forcircuitry 35 to determine further conditions of the circuit 10 prior tocommanding to decouple the power input 24 from the power output 40. Inan example, circuitry 35 may include additional logic components todetermine if the power input 24 is on or off, and, further, give weightto that determination in calculating the appropriate signal to generateas its output.

1. A fail open circuit, comprising: an input voltage coupled to anoutput load through a control switch; a control signal regulating theconduction state of said control switch; and fail open circuitryreceiving said control signal and a signal indicative of the energydelivered to said load, and adapted to determine if said control switchis in the proper conduction state based on said control signal, saidfail open circuit further adapted to decouple said input voltage fromsaid load using said energy delivered to said output load if saidcontrol switch is in an improper conduction state.
 2. A circuit asclaimed in claim 1, further comprising control circuitry generating asecond control signal to decouple from said load when said controlswitch is in said improper conduction state.
 3. A circuit as claimed inclaim 2, wherein said fail open circuitry comprises fuse circuitrycoupled to a second switch, said fuse circuitry being disposed betweensaid input voltage and said load, and wherein said control circuitryreceiving said control signal and said signal indicative of the energydelivered to said load and generating said second control signal tocontrol the conduction state of said second switch, wherein if saidcontrol switch is in an improper conduction state, said second switchdiverts energy at said load through said fuse circuitry therebydecoupling said load from said input voltage.
 4. A circuit as claimed inclaim 3, wherein said control circuitry including an inverter receivingsaid control signal and generating an inverted control signal, and anAND gate receiving said inverted control signal and said signalindicative of the energy delivered to said load; and generating saidsecond control signal.
 5. A circuit as claimed in claim 3, wherein saidfuse circuitry comprises a heating resistor and a thermal fuse, whereinsaid heating resistor heating said fuse and blowing said fuse when saidsecond switch conducts thereby decoupling said load from said inputvoltage.
 6. A circuit as claimed in claim 5, wherein said thermal fusebeing coupled to said control switch, and said thermal fuse blowing whencurrent through said switch exceeds said fuse's current rating.
 7. Acircuit as claimed in claim 5, wherein said thermal fuse being inthermal proximity to said control switch, and said fuse blowing whentemperature of said switch exceeds said fuse's temperature rating.
 8. Amethod for fail-open circuit operation, comprising the steps of:coupling an input voltage to a load through a control switch; regulatingthe conduction state of said control switch with a control signal;determining if said control switch is in a proper conduction state basedon said control signal and the energy delivered to said load; decouplingsaid input voltage from said load using said energy delivered to saidload if said control switch is in an improper conduction state.
 9. Amethod as claimed in claim 8, further comprising the steps of:generating a signal indicative of the energy delivered to said load;inverting said control signal; ANDing said inverted control signal andsaid signal indicative of the energy delivered to said load, andgenerating a second control signal; coupling fuse circuitry to said loadand a second switch; controlling said second switch with said secondcontrol signal so that if said control switch is in an improperconduction state, said energy delivered to said load is transmittedthrough said fuse circuitry and said second switch thereby decouplingsaid input voltage from said load.
 10. A method as claimed in claim 9,further comprising the steps of: using said energy through said fusecircuitry to heat a thermal element; placing said thermal element inthermal proximity to a thermal fuse; and blowing said thermal fuse usingheat transmitted from said thermal element thereby decoupling said inputvoltage from said load.
 11. A method as claimed in claim 10 furthercomprising the steps of: placing said thermal fuse in thermal proximityto said control switch; and blowing said fuse when the temperature ofsaid switch exceeds said fuse's temperature rating.
 12. A fail opencircuit, comprising: an input voltage; an output load; a control switchcontrollably coupling said input voltage to said load; a control signalregulating the conduction state of said control switch; fail opencircuitry receiving said control signal and a signal indicative of theenergy delivered to said load, and adapted to determine if said controlswitch is in the proper conduction state based on said control signal;fuse circuitry disposed between said input voltage and said load, andwherein said fail open circuitry receiving said control signal and saidsignal indicative of the energy delivered to said load, wherein if saidcontrol switch is in an improper conduction state, said fail opencircuitry diverting said energy at said load through said fuse circuitrythereby decoupling said load from said input voltage.
 13. A circuit asclaimed in claim 12, wherein said fail open circuitry comprises a secondswitch, and wherein said control circuitry receiving said control signaland said signal indicative of the energy delivered to said load andgenerating said second control signal to control the conduction state ofsaid second switch, wherein if said control switch is in an improperconduction state, said second switch diverts energy at said load throughsaid fuse circuitry thereby decoupling said load from said inputvoltage.
 14. A circuit as claimed in claim 13, wherein said controlcircuitry including an inverter receiving said control signal andgenerating an inverted control signal, and an AND gate receiving saidinverted control signal and said signal indicative of the energydelivered to said load, and generating said second control signal.
 15. Acircuit as claimed in claim 13, wherein said fuse circuitry comprises aheating resistor and a thermal fuse, wherein said heating resistorheating said fuse and blowing said fuse when said second switch conductsthereby decoupling said load from said input voltage.
 16. A circuit asclaimed in claim 15, wherein said thermal fuse being coupled to saidcontrol switch, and said thermal fuse blowing when current through saidswitch exceeds said fuse's current rating.
 17. A circuit as claimed inclaim 15, wherein said thermal fuse being in thermal proximity to saidcontrol switch, and said fuse blowing when temperature of said switchexceeds said fuse's temperature rating.